This invention relates to a pro,be apparatus for measuring electrical signals in an electrical circuit comprising at least one electrical contact for contacting a selected point of the electrical circuit, said electrical contact being coupled to a first terminal of a switch responsive to electromagnetic radiation, the probe apparatus further comprising means for generating electromagnetic radiation and applying it to said switch for switching the same. In particular, the present invention relates to such a probe apparatus for measuring high-frequency signals in integrated circuits and other electronic circuits. The invention further relates to a method for measuring electrical signals in integrated and other circuits.
A probe apparatus according to the introductory paragraph is known from EP-A 0 324 110, which corresponds to U.S. Pat. No. 4,851,767 (Jul. 25, 1989). In this document a probe is disclosed in which an electrical contact for contacting a point in the circuit under test is directly coupled to electrically conductive wiring of the probe where a signal is sampled via a photoconductive gap responsive to radiant energy. The known device comprises a laser and an optical system for transmitting light from the laser onto the photoconductive gap. As the laser pulses can be made very short, a probe for high speed measurement is provided.
The known document leaves the effects of signal detection on the circuit under test out of consideration. However, the load consisting of the probe and the circuitry immediately following it may influence the signal levels in the circuit and consequently the accuracy of the measurement. For example, an ohmic load of the photoconductive switch causes a current to be drawn from the test point and a change of the voltage level at that point.